Most cells have on their surfaces receptors that can specifically interact with molecules in the surrounding solution. Molecules that bind to receptors are collectively known as ligands. For example, the ligand may be a peptide hormone such as insulin and the receptor the “insulin receptor.” Alternatively, the ligand might be a polysaccharide derived from the cell wall of a bacterium, and the receptor an immunoglobulin molecule on the surface of a lymphocyte, a type of white blood cell. If the ligand is a large molecule, it generally will have multiple sites, or functional groups, of which f can simultaneously interact with cellular receptors. Receptor molecules may also be multivalent and contain g, g ≥ 1, identical ligand binding sites. For example, immunoglobulin molecules, which act as receptors on lymphocytes, basophils, and mast cells, contain two identical binding sites. When multivalent ligands interact with multivalent cell surface receptors, receptor-ligand aggregates form on the cell surface. The formation of such aggregates, as shown by a large number of biological examples, appears to be of primary importance in determining how cells respond to the presence of molecules in their environment. As we show later in this monograph, the size of receptor-ligand aggregates and their speed of formation depend on the ligand concentration in the surrounding solution (as well as other factors). Thus by sensing the size and/or rate of formation of receptor-ligand aggregates on its surface, a cell can roughly determine the concentration of ligand in its environment.
KeywordsHistamine Release Receptor Cluster Cluster Size Distribution Aggregation Phenomenon Free Site
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